1. Field of the Invention
The present invention relates to an optical information reproducing method which reproduces multivalued information by using three or more values, and, particularly, relates to a method for detecting phase error information for a PLL (phase-locked loop), which produces a reproduction clock to be used when recorded information is reproduced.
2. Description of the Related Art
In recent years, an optical memory industry has been expanded. Various optical memory media have been developed, such as a reproduction-only CDs and DVDs, a write-once type recording medium using a metallic film and a pigment-based recording material, and further, a rewritable type recording medium using a photomagnetic material and a phase-changeable material. The application also has been widened from consumer products to an external memory of a computer.
Furthermore, research and development have proceeded on the identification for a recording capacity of the recording medium. One technology for minimizing a light spot related to the recording and reproduction of information is to change a wavelength of a light source from red (650 nm) to blue-violet (405 nm). Another technology is to change a numerical aperture of an objective lens from 0.6 or 0.65 to 0.85. On the other hand, a technology has been proposed which more efficiently records and reproduces information while using the same size of a light spot, but one having a multi-value.
For instance, a method is proposed in Japanese Patent Application Laid-Open No. H05-128530, as a method of a multi-value recording and reproducing technology. The above Japanese patent publication discloses a method of recording multivalued information on an information track of an optical information recording medium by combining a length of an information pit in a track direction with a shift amount for a light spot for reproducing the information pit in the track direction. This Japanese patent publication also discloses a method of reproducing the multivalued information pit by using the correlation between a previously-learned detecting signal and a detecting signal provided from the light spot, when reproducing the multivalued-recorded information pit.
In addition, in ISOM (International Symposium on Optical Memory) 2003 (Write-once Disks for Multi-level Optical Recording: proceedings Fr—Po—), which is an international institute in a research field of an optical disk, the following technology is presented. Specifically, the technology records and reproduces eight-level multi-value information onto and from an optical disk while using a light source of blue-violet (405 nm), an optical system having an NA of 0.65, an optical disk with a track pitch of 0.46 μm, and a region for recording one information pit (hereafter, referred to as a cell), which is virtually arranged and has a width of 0.26 μm in a track direction.
In order to select an eight-level information pit, for instance, a length of a cell in a track direction (direction (A) in the figure) is divided into sixteen equal parts, as shown in FIG. 20; and a level 0 is set for no recorded information pit, level 1 is set at a width corresponding to a 2/16 cell, level 2 at a width corresponding to the 4/16 cell, level 3 at a width corresponding to the 6/16 cell, level 4 at a width corresponding to the 8/16 cell, level 5 at a width corresponding to the 10/16 cell, level 6 at a width corresponding to the 12/16 cell, and level 7 at a width corresponding to the 14/16 cell.
The thus selected information pit is recorded on a track at random, and a photodetector receives light reflected from the pit, and detects the light amount. Suppose that a reproduced signal is sampled from the obtained multivalued information pit when a center of a light spot comes to the center of a cell in a track direction. Then, amplitude of the reproduced signal at each level shows such a distribution as shown in FIG. 21.
The technology also employs a configuration in which a clock for sampling information detects phase error information by reproducing a predetermined pit row inserted at predetermined intervals while the information has been recording, in the above step, and produces the phase error information in a phase-locked loop circuit (PLL circuit).
FIG. 22 illustrates an example which has a pit row for detecting a phase error inserted between other rows. In the figure, reference characters (A) to (I) denote recorded information rows, and reference character (A) denotes the pit row for PLL lock-in, which is positioned at the head of recorded data. The pit row for PLL lock-in is adopted, in many cases, no matter what the multivalued recording is. Reference character (B) denotes a learning pit row for obtaining a reproduced signal of the known pit row.
Reference characters (C), (E), (G) and (I) denote encoded user information. Reference characters (D), (F) and (H) denote pit rows for detecting a phase error for a PLL, which produces a reproduction clock. For instance, an edge is extracted through a digitalization process with the use of a level-slicing technique. Then, a phase comparator detects information on the phase error between the edge and the reproduction clock. The phase error information is used for the PLL processing of the reproduction clock. In multivalued recording, edges periodically appear regardless of a recorded pit size. Accordingly, in order to obtain phase information with a high signal-to-noise ratio, it is desirable to control the pit row for detecting the phase error, so that the minimum level and the maximum level can alternately appear.
Let us return to the description in FIG. 21. When a state of level 0 having no information recorded in a pit continues, the reproduced signal level is standardized to be “1”, and when the information of level 7 is continuously recorded in pits, the reproduced signal level is standardized to be “0”.
The value of the reproduced signal corresponding to each level has an amplitude, because the value is affected (intersymbol interference) by information written in the pits in front and back of the noted information pit. As is shown in FIG. 21, when an amplitude distribution of a reproduced signal in a certain level overlaps with that in the next level, the reproduced signal cannot be separated/detected by using a fixed threshold value.
A method for solving the problem is disclosed in ISOM 2003 (Write-once Disks for Multi-level Optical Recording: proceedings Fr—Po-04). The separating/detecting method specifically includes making the optical recording system carry out: reading a reproduced signal from a pit row containing a previously known value in a noted information pit and a value in information pits in front and back, memorizing it (learning), and comparing the reproduced signal obtained from the real information pit with a recorded value (recognition of correlation). The method provides a recording density of about 16 Gbit/inch2.
When the above-described method for recording and reproducing the information of a multivalued level aims to obtain a phase error signal at a sufficient precision in order to produce a reproduction clock for detecting the reproduced signal level, the method shows the following problems that the method needs to insert a sufficiently large pit row with a predetermined length at a predetermined spacing, so as to be capable of detecting the phase information which has been digitized by using a fixed slicing technique.
The inserted pit row for detecting the phase error then lowers the information efficiency of a recording format in an information recording medium, and hinders row density in multivalued recording from being improved.